The present invention relates generally to gas exchange measurement systems, and more particularly to open photosynthesis measurement systems having an optimized flow configuration to minimize errors resulting from gas diffusion.
Systems for measuring plant photosynthesis and transpiration rates can be categorized as open or closed systems. For open systems, the leaf or plant is enclosed in a chamber, and an air stream is passed continuously through the chamber. CO2 and H2O concentrations of chamber influent and effluent are measured, and the difference between influent and effluent concentration is calculated. This difference is used, along with the mass flow rate, to calculate photosynthesis (CO2) and transpiration (H2O) rates. For closed systems, the leaf or plant is enclosed in a chamber that is not supplied with fresh air. The concentrations of CO2 and H2O are continuously monitored within the chamber. The rate of change of this concentration, along with the chamber volume, is used to calculate photosynthesis (CO2) and transpiration (H2O) rates.
In both open and closed systems, it is important that the leaf or plant be the only source or sink of both CO2 and H2O. CO2 or H2O concentration changes not caused by the plant are a measurement error. These errors can be empirically compensated, for example as described in the LI-COR Biosciences LI-6400 User Manual (pp. 4-43 thru 4-48; included herein as Appendix A). Some instrument users may not understand the significance of these corrections, and neglect them.
The ability to regulate CO2 and H2O concentrations in and around the sample is critical to accurate photosynthesis and transpiration measurements, and is a key function of the leaf chamber. The ideal chamber must not impact the dynamic control or measurement of CO2 and H2O concentrations. A well known artifact of chamber construction is the uncontrolled release or retention of CO2 or H2O by chamber surfaces. This phenomenon is generally known as sorption, and describes both adsorption and desorption. Adsorption is the retention of H2O or CO2 molecules on the chamber surfaces. Desorption is the release of H2O or CO2 molecules from the chamber surfaces. Both open and closed systems contain a circuit of pneumatic components (e.g., pumps, valves, chambers, tubing, analyzers, etc.). When CO2 and H2O concentrations are dynamically changing, sorption on these components can provide an apparent CO2 or H2O source and/or sink. Under steady-state conditions, sorption is not an active source or sink, and parasitic CO2 and H2O sources and/or sinks can be attributed to bulk leaks and diffusion.
Bulk leaks are driven by pressure differentials between the system and the ambient environment. Proper system design and construction, along with inherently low operating pressures, generally minimize parasitic sources and sinks due to bulk leaks. Diffusion is driven by constituent gas (CO2 and H2O) concentration gradients between the system and ambient environment. Any time constituent gas concentrations inside the system are significantly different than ambient conditions, the diffusion potential increases. Metals, in nearly any practical working thickness, generally provide an outstanding diffusion barrier to gases. Practically, however, nonmetallic materials are usually required. For example, to provide a seal between metallic materials, gaskets and O-rings are used. Flexible tubing which connects the sensor head to other system components is an example of functional capabilities which cannot be reasonably achieved with metals.
In open photosynthesis systems, a conditioned air stream is typically split into two streams. FIG. 1a illustrates the flow path in such an open system where the flow is split at the console, remote from the sensor head, and flows to the sensor head via two separate paths. The first flow path (known as reference) passes through a gas analyzer (e.g., Infra-Red Gas Analyzer or IRGA) which measures constituent gas concentrations (CO2 and H2O). The second flow path (known as sample) passes through a sample chamber (leaf chamber) in which gas exchange occurs. This second sample flow path exits the chamber and enters a second gas analyzer (e.g., IRGA). The difference between the sample and reference gas concentrations is used to calculate photosynthesis (CO2) and transpiration (H2O). As photosynthesis and transpiration measurements are based on concentration differences in these two gas streams, the accuracy in measuring the difference is more critical than measuring the absolute concentration of either. Diffusive parasitic sources and/or sinks present in the tubing, connectors, and fittings that supply the head with the sample and reference gas streams can compromise measurement accuracy.
In practice, it is nearly impossible to fully eliminate parasitic sources and sinks due to diffusion. Therefore it is desirable to provide systems and methods that minimize the impact of diffusion and/or sorption and that help overcome the above and other problems.
Additionally, chambers used in photosynthesis and transpiration measurements typically incorporate mechanisms for regulating sample temperature. Heat transfer to and from the sample is accomplished either through radiative heat transfer between the sample and chamber walls, convective heat transfer between the chamber air and the sample, or both. Inevitably, parasitic heat transfer occurs between the chamber and the surrounding environment, thereby compromising the efficiency of sample temperature control. It is also therefore desirable to provide solutions that allow for more energy efficient temperature control.
Moreover, in many photosynthesis measurement systems, such as portable photosynthesis measurement systems, the size and weight of the chamber are primary ergonomic considerations. For portable systems, the chamber is manually transported, placed, and sometimes held in place for the duration of the photosynthesis measurement. It is therefore also desirable to provide light-weight and cost-effective photosynthesis measurement systems.